cold flow

Cold flow is the slow, irreversible deformation of a material under constant stress at ambient or moderately elevated temperature, without the material reaching its melting point or even approaching it. Unlike creep at high temperature, which is driven by atomic diffusion and grain boundary sliding, cold flow occurs through mechanisms such as dislocation movement, stress-induced phase transformation, or in polymers, molecular chain relaxation. The material yields plastically over time even though the applied stress may be well below the conventional yield strength.

In polymers, cold flow is the primary concern. Plastics, elastomers, and composites all exhibit time-dependent deformation under load at room temperature. A polyethylene pipe under sustained internal pressure will gradually expand in diameter; a polycarbonate lens mount will sag under its own weight over months or years; silicone gaskets will compress and lose sealing force. The rate depends on polymer type, crystallinity, temperature, stress level, and humidity. Polyester resins flow faster than epoxies; amorphous materials flow more than semi-crystalline ones.

Nonferrous metals and alloys

Low-melting-point metals and alloys experience cold flow at room temperature. Lead, tin, and their alloys are notorious offenders. A lead solder joint under mechanical vibration or thermal cycling will deform plastically over time. Aluminium alloys, particularly those used in aerospace applications, can creep noticeably at modest temperatures if the stress is high enough. The effect becomes worse as temperature rises but remains significant at ambient conditions for susceptible materials. Annealed copper will flow under sustained tension; strain-hardened copper resists it better.

Cold flow becomes problematic in applications where dimensional stability or structural integrity must be maintained. Bearing bushings made from bronze or aluminium will wear into ellipses under load. Precision measuring blocks or gauge blocks will drift out of tolerance over decades if the load is constant. Seals and gaskets lose effectiveness as the material compresses. In electronics, tin whiskers can form and grow over years due to stress relief in pure tin coatings. Material selection and prestressing strategies (such as applying initial compression) can mitigate the problem.

The term "cold" is relative: it simply means the process occurs without external heating and without approaching the material's melting temperature. In industrial settings, cold flow is often studied through tensile creep tests conducted at fixed stress and ambient temperature over days or weeks, with periodic measurements of dimensional change. It is distinct from stress relaxation, in which stress decreases under constant strain. Both phenomena matter in long-service applications such as gaskets, seals, fasteners, and load-bearing polymer components.